Millions of people throughout the world, including as many as twenty million Americans are edentulous (toothless) and rely on full dentures for function and esthetics. In fabricating dentures, it is crucial for the dentist to assure that the upper denture is well-fitted and secure. This is important not only in speech and mastication, but also in avoiding the embarrassment of a loose or falling denture.
A secure denture is usually accomplished by the denture adapting well to the gum tissue, and in the case of the upper member of dentures, through the use of a posterior palatal seal (PPS). The combination of a well-fitting denture and a well-fitting PPS will create suction or vacuum once the upper denture is seated in the mouth. A PPS is a thickened posterior portion of the upper denture border extending across the palate from the left to the right maxillary tuberosity. The increased thickness of the denture along a narrow border adds "pressure" on the tissue and maintains a vacuum seal. Without this sealing effect created by the PPS, the upper denture "leaks" (the vacuum between the denture and the roof of the mouth is reduced or lost) and the denture loosens as the wearer talks, chews or swallows.
A PPS is beneficial and considered necessary by many dentists because the border of the upper denture at the posterior palate area rests on the junction between the hard and soft palate. Unless there is additional pressure against that area of the palate, there will be loss of suction during function. Anatomically, the key effective sealing area corresponds to the junction between the hard and soft palate. Among dentists, this is also called the "Ah-Line." In locating how far a denture should extend backwards, the dentist may ask the patient to repeat the "ah" sound. In doing so, the soft palate vibrates and the hard palate does not vibrate so that the demarcation between the soft and hard palate is discernible. This is typically where the denture border will lie. The junction between the hard and soft palate where the denture border lies is neither a straight line nor of uniform consistency. It is harder in the midline and soft on both sides of the midline. A well constructed PPS, therefore, has to reflect the anatomy and features of that part of the mouth.
Presently, the most common method of fabrication of the PPS requires either a dentist or a laboratory technician to carve an indentation channel on a plaster model obtained from an impression of the toothless mouth. The impression material is typically alginate, silicon, zinc oxide-eugenol or a rubber base material. The dentist normally takes an impression of the edentulous mouth and often prepares a plaster (or dental stone) model from the impression by pouring wet plaster onto the impression. Alternatively, the impression is boxed and sent to a dental lab for preparation of a plaster or dental stone model. When the plaster has set hard, it is separated from the impression. Either a dentist or a lab technician then carves an indentation on the plaster model where the PPS will be. The depth of carvings for the PPS preferably corresponds to the anatomical qualities of the area of the palate mentioned above. That is, to be an effective seal, less must be carved away from the midline where the palate tissue is hard and more on both sides of the midline where the palate tissue is softer. The carved portion is gradually deepened as it reaches the posterior border. If all goes well, the carving results in a "Cupid's Bow" like appearance with the "string" part of the bow at the denture border and the "serpentine bow" towards the front part of the mouth. Shallowest at the "serpentine bow", it deepens going towards the "string" of the bow (at the border of the denture) with the deepest on both sides of the midline and shallower at the midline of this "bow" and also the two ends of this "bow." If the "carver" lacks skill or if short cuts are taken, a simple groove might be carved or a denture might be formed without a PPS of any kind.
Carving methods are taught in practically every dental school and various shapes of PPS are practiced by most denture technicians. The carving method for forming a good PPS is tedious, time-consuming and the results are inconsistent. Some technicians simply carve a trench instead of a bow shape or another approximation of a "Cupid's bow" shape either because it saves time, because they lack a high degree or skill, or because they do not understand oral anatomy and physiology. The method of hand carving a "Cupid's bow" continues to be described and recommended by professors at reputable dental schools and major international journal articles, as for example in Quintessence, Int., 1993; 24: 753-755.
Another less prevalent method of forming a PPS is termed a functional method or a waxing method. This method if currently more tedious and more time-consuming than the carving method. During an impression visit, the dentist applies a wax material on the impression in the poster palatal area of the impression. Again, a Cupid's bow shape provides an effective seal. A special wax is used which liquifies at mouth temperature and solidifies at room temperature. The wax is carefully shaped as by dabbing, brushing or otherwise placing it bit-by-bit and smoothing it onto the impression along the identified "Ah line." The impression is reinserted into the patient's mouth and held for several minutes to allow partial liquification and plastic deformation into and along the actual functional border between the hard and soft palate. The impression is then removed and carefully boxed to ship it to the dental lab for pouring of a plastic or a dental stone mold from which a denture will be molded. The wax must be maintained in a solidified condition during pouring so that a PPS channel will be formed in the mold. This channel in the mold will result in a raised PPS in the molded denture.
The process of forming a PPS on an existing denture has many steps and is time-consuming. The technician has to lubricate the denture; pour up a plaster model onto the denture; separate the model from the denture; carve the needed void for the PPS from the plaster model as described earlier; lubricate the model so that new denture material will not stick to the model; place new denture material into the carved void between the model and the denture; apply pressure so there is intimate contact between the new material and the denture base; wait for chemical bonding and curing of the new material; separate the denture from the model; and smooth and polish it for delivery to wearer.